Internal load for a travelling wave tube using a folded-waveguide slow-wave structure

Abstract

A folded-waveguide slow-wave structure equipped with an internal load, includes a central plate comprising a rectilinear beam tunnel of same direction as the longitudinal axis of the central plate, and a serpentine-shaped folded slit having its folds in the direction of the width of the waveguide; a lower plate and an upper plate closing the waveguide, the plates being placed on and under the central plate, respectively; at least one groove of cross section that may be variable, produced along the longitudinal axis of the waveguide, in at least one face internal to the waveguide of the lower plate, the upper plate or the central plate, and at least partially comprising a lossy material; in order to form a closed slow-wave structure through which propagates a hybrid slow wave the amplitude of which is attenuated by at least 20 dB between the start and the end of the portion of the one or more grooves containing a lossy material.

Claims

1. A folded-waveguide slow-wave structure equipped with an internal load, comprising: a central plate comprising a rectilinear beam tunnel of same direction as the longitudinal axis of the central plate, and a serpentine-shaped folded slit having a plurality of folds in the direction of the width of the waveguide; a lower plate and an upper plate closing the waveguide, said plates being placed under and on the central plate, respectively; at least one groove of cross section that may be variable, produced along the longitudinal axis of the waveguide, in at least one face internal to the waveguide of the lower plate, the upper plate or the central plate, and at least partially comprising a lossy material; in order to form a closed slow-wave structure through which propagates a hybrid slow wave the amplitude of which is attenuated by at least 20 dB between the start and the end of the portion of the at least one groove of cross section containing a lossy material.

2. The folded-waveguide slow-wave structure according to claim 1, wherein the lossy material is a lossy dielectric.

3. The folded-waveguide slow-wave structure according to claim 2, wherein at least one groove has a constant cross section and comprises an amount of a given lossy dielectric that increases as the abscissa increases along the axis of the waveguide oriented in the direction of wave propagation.

4. The folded-waveguide slow-wave structure according to claim 2, wherein at least one groove has a cross section that remains constant or that increases as said abscissa increases and is filled with lossy dielectric the level of microwave losses of which increases as said abscissa increases.

5. The folded-waveguide slow-wave structure according to claim 1, wherein the lossy material is a layer of a mixture of metals chosen from iron, nickel, molybdenum and titanium, at least partially covering the internal surface of a groove.

6. The folded-waveguide slow-wave structure according to claim 5, wherein at least one groove has a cross section the edge length of which remains constant and comprises an amount of a layer of said same mixture of metals that increases as said abscissa increases.

7. The folded-waveguide slow-wave structure according to claim 5, wherein at least one groove has a cross section the edge length of which remains constant or increases as said abscissa increases and comprises a layer of said same mixture of metals.

8. A process for manufacturing a folded-waveguide slow-wave structure equipped with an internal load, comprising: drilling in a central plate a rectilinear beam tunnel of same direction as the longitudinal axis of a central plate, and a serpentine-shaped folded slit having a plurality of folds in the direction of the width of the waveguide; producing at least one groove of cross section that may be variable, along the longitudinal axis of the waveguide, in at least one face internal to the waveguide of a lower plate, of an upper plate or of the central plate, and at least partially comprising a lossy material; and placing the lower plate and the upper plate closing the waveguide, under and on the central plate, respectively; so as to form a closed slow-wave structure through which propagates a hybrid slow wave the amplitude of which is attenuated by at least 20 dB between the start and the end of the portion of the at least one groove of cross section containing a lossy material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood on studying a few embodiments that are described by way of completely nonlimiting example and that are illustrated by the appended drawings, in which:

(2) FIG. 1 schematically illustrates a slow-wave structure or delay line for a travelling wave tube comprising three sections, according to the prior art;

(3) FIG. 2 schematically illustrates a delay line comprising a helix held in an envelope by three dielectric rods, according to the prior art;

(4) FIG. 3 schematically illustrates the attenuation on a rod of a delay line of the type in FIG. 2, as a function of the thickness of the deposit of the material generating the high microwave losses, according to the prior art;

(5) FIG. 4 schematically illustrates a folded-waveguide delay line comprising a matched load in a straight waveguide parallel to the beam tunnel, according to the prior art;

(6) FIG. 5 schematically illustrates a folded-waveguide delay line comprising a matched load in a straight waveguide parallel to the beam tunnel and folded toward the cells of the line, according to the prior art;

(7) FIG. 6 schematically illustrates a folded-waveguide delay line comprising a matched load in a straight waveguide perpendicular to the beam tunnel, according to the prior art;

(8) FIG. 7 schematically illustrates a folded-waveguide delay line that is interrupted by a lossy dielectric block of a geometry determined to minimize reflections from the load, according to the prior art; and

(9) FIGS. 8, 9 and 10 schematically illustrate an overview and cross-sectional views of a folded slow-wave structure for a travelling wave tube, according to one aspect of the invention.

(10) In all of the figures, elements referenced with identical references are similar.

DETAILED DESCRIPTION

(11) In the present description, the described embodiments are completely non limiting, and features and functions that are well known to those skilled in the art are not described in detail.

(12) FIG. 8 schematically shows a folded-waveguide slow-wave structure for a travelling wave tube, which is equipped with an internal load comprising: a central plate 20 comprising a rectilinear beam tunnel 21 of same direction as the longitudinal axis z of the central plate 20, and a serpentine-shaped folded slit 22 having its folds in the direction of the width of the waveguide; a lower plate 23 and an upper plate 24 closing the waveguide, said plates being placed on and under the central plate 20, respectively; at least one groove 25 of cross section that may be variable, produced along the longitudinal axis z of the waveguide, in at least one face internal to the waveguide of the lower plate 23, the upper plate 24 or the central plate 20, and at least partially comprising a lossy material; in order to form a hybrid slow-wave structure such that the amplitude of a wave is attenuated by at least 20 dB between the start and end of the portion of the one or more grooves containing a lossy material.

(13) In other words, the present invention consists in gradually introducing electromagnetic losses into the folded-waveguide delay line in order to avoid an abrupt transition between the periodic line and a rectangular waveguide, or between the periodic line and a dielectric block, equivalently to the increase in the thickness of the graphite deposit on the rods of a helix delay line known from the prior art.

(14) To do this, the folded-waveguide delay line is coupled to another transmission line that generates high losses, and the coupling between the two lines increases in the direction of propagation of the wave. If a cell is defined as the volume bounded by two planes perpendicular to the axis of the beam and separated by one pitch (i.e. the distance between two straight waveguides separated by one bend), the amplitude of the wave decreases from one cell to the next.

(15) In the example of FIG. 8, two grooves 25 of variable cross section that increases with the abscissa of the z-axis of the waveguide, which grooves are in the present case symmetric with respect to the midplane of the central plate 20, are produced in the face internal to the waveguide of the lower plate 23 and in the face internal to the waveguide of the upper plate 24, and are filled with lossy dielectric, such as a ceramic (alumina, beryllium oxide, aluminum nitride) sintered with elements that generate microwave losses (carbon, iron, titanium, etc.).

(16) The high-electromagnetic-loss transmission line may be machined in the lower plate 23 and/or upper plate 24, which are brazed to the central plate 20 in which the serpentine 22 is machined in order to form the folded-waveguide delay line. It is therefore a question of a waveguide recessed by machining into the lower plate 23 and/or upper plate 24. It may also be partially or completely machined in the central plate 20.

(17) In the example of FIG. 8, the grooves of variable cross section produced in the face internal to the waveguide of the upper plate 24 cannot be seen.

(18) FIG. 9 shows a cross-sectional view of a folded slow-wave structure for a travelling wave tube, according to one aspect of the invention.

(19) FIG. 10 shows various cross sections of the example in FIG. 9.

(20) As a variant, any embodiment comprising at least one groove 25 of cross section that may be variable (variable or constant), produced along the longitudinal axis z of the waveguide, in at least one face internal to the waveguide of the lower plate, the upper plate or the central plate, and at least partially comprising a lossy material, is possible.

(21) To generate these electromagnetic losses, it is possible to partially or completely fill the grooves 25 with one or various lossy dielectrics, or to deposit one or various lossy materials on the walls, so that along said longitudinal axis oriented in the direction of propagation of the wave, as the abscissa increases, the amplitude of the wave is attenuated by 20 dB between the start and end of the load.

(22) The following are the most explicit cases.

(23) At least one groove 25 may have a constant cross section and comprise an amount of a given lossy dielectric that increases as said abscissa increases.

(24) As a variant, at least one groove 25 may have a cross section that remains constant or increases as said abscissa increases and be full of lossy dielectric the microwave loss level of which increases as the abscissa increases.

(25) As a variant, at least one groove 25 may have a cross section the edge length of which remains constant and comprise an amount of a layer of a given mixture of metals chosen from: iron, nickel, molybdenum and titanium, at least partially covering the internal surface of a groove that increases as the abscissa increases.

(26) As a variant, at least one groove 25 may have a cross section the edge length of which remains constant or increases as said abscissa increases and comprise a layer of a given mixture of metals chosen from: iron, nickel, molybdenum and titanium.

(27) The broadside of the guide machined in the lower and upper plates determines the aperture in the folded-waveguide line, and therefore the coupling between the two transmission lines. A lossy guide of small height may correspond to a guide operating sub cut-off frequency and therefore to a waveguide that prevents energy from propagating into the lossy guide. In this case, the waveguide behaves like a damped resonant cavity coupled to the folded waveguide.

(28) The process for manufacturing such a folded-waveguide slow-wave structure equipped with an internal load, comprises steps consisting in: drilling in a central plate 20 a rectilinear beam tunnel 21 of same direction as the longitudinal axis z of a central plate 20, and a serpentine-shaped folded slit 22 having its folds in the direction of the width of the waveguide; producing at least one groove 25 of cross section that may be variable, along the longitudinal axis z of the waveguide, in at least one face internal to the waveguide of a lower plate 23, of an upper plate 24 or of the central plate 20, and at least partially comprising a lossy material; placing the lower plate 23 and the upper plate 24 closing the waveguide, under and on the central plate 20, respectively;

(29) so as to form a closed slow-wave structure through which propagates a hybrid slow wave the amplitude of which is attenuated by at least 20 dB between the start and end of the portion of the one or more grooves containing a lossy material.

(30) The plates, which are generally parallelepipedal, may be produced using conventional laminating or milling processes.

(31) The beam tunnel 21 may be produced by electrical discharge machining (EDM), and the slit 22 in the central plate may be produced by wire EDM.

(32) The grooves 25 may be produced by micro-milling or by EDM.